Hoisting control system



I v w. SCHAELCHLIN Er AL ,768-

HOI STING CONTROL SYSTEM Filed Sept. 8, 1938 2 Sheets-Sheet 1 WITNESSES:INVENTORS ATTORNEY 1939- w. SCHAELCHLIN ET AL ,768

HOI STING CONTROL SYSTEM Filed Sept. 8, 1938 2 Sheets-Sheet 2 WITNESSES:f

S INVENTORS h/a/fer Schae/ch/bv and WW a Agrf Mahoke.

ATTORNEY Patented Dec. 26, 1939 PATENT OFFICE HOISTING CONTROL SYSTEMWalter Schaelchlin and Kurt Mahnke, Wilkinsburg, Pa., "assignors toWestinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., acorporation of Pennsylvania Applicationseptember 8, 1938, Serial No.228,933

17 Claims.

Our invention relates to motor control systems, and particularly tosystems of controlv for controlling motors operating hoists such ascargo winches, mining hoists, cranes, etc.

Generally, our invention is intended to provide a maximum degreeofflexibility in controlling a motor operating a cargo winch, so that theentime range of speed may be readily kept under control fromno-load tofull-load both in the hoisting direction and in the lowering direction.a

One object of our invention is to provide a system of control for acargo winch, with provisions for controlling the speed of the hoist forboth lowering operation and hoisting operation from a minimum to amaximum speed over a range of load varying from a minimum to a maximum.

Another object of our invention is to provide a system of control for avw'nch to achieve slow ing a motor, as was heretofore necessary.

A further object of our invention is to achieve slow lowering speeds athigh efficiency.

Another object of our invention is to provide a system embodying noveldynamic braking to achieve better control of low speeds than heretoforepossible.

Another object is to provide a system to establish a high startingtorque against sluggish equip- 39 ment or unloaded hook in the loweringdirection.

Another object of our invention is to provide a control system that willpermit the employment gf a novel highly efiicient dynamic brakingcircuit.

35 Another object of our invention is to provide a control system for acargo winch, to provide extreme flexibility of operation and sosafeguarded that the master controller may be moved suddenly in anydirection irom any position without w danger to the motor and theequipment.

Generally, our invention includes a conventional series motor whichoperates as a series motor during hoisting and as a shunt motor duringlowering, but which motor is provided with spe- 45 cial resistor controland has an additional separately excited field winding, the excitationof which is controlled in special relation to the direction of operationof the motor and the load on the motor.

so During lowering, as stated, the motor series field is operating as ashunt field and the separately excited-field is in efiect an additionalshunt field. We secure high empty hook speed in the lowering directionfor the last orhighest speed' ,5 point by a suitable control of the twofield wind- 0 setting down speed to avoid the need of inch-I ings. Thisadvantage and the many other advantages will become more apparent from astudy of the following specification when taken in conjunction with thedrawings, in which:

Figure 1 is a circuit diagram of the entire congraphically therelationship between the m'otor speed and the load on the hoist hook,for the various positions of the master controller in the hoisting andthe lowering positions.

Referring to Fig. 1, reference characters 0 and 1' designate thepositive and negative buses, respectively, of the source of energy and ldesignates schematically the main switch. M designates the motor foroperating the cargo winch. This motor M hasa series field winding S, anarmature winding A and a shunt field winding SH. The series fieldwinding S, like in prior art devices, is, during lowering, connected asa shunt field winding but unlike prior art devices is provided with aspecial control coacting with special control for a shunt field SH whichcontrol made it possible to usea separate or extra shunt field forsecuring novel operations, not heretofore possible with cargo winchcontrol.

The control for the two field windings, among other elements, includesthe starting resistor R including the resistor sections, a load relay,and a plurality of specially controlled resistor shunting contactors, orrelays, 43, 66, 70 and T4, and a master controller C, having five, ormore, lowering and live, or more, hoisting positions.

The motor is provided with a dynamic braking resistor so arranged andcontrolled to effect dynamic braking for overhauling loads, or inertiaload at stopping for stopping, that is, when the controller 0 is in theoff position, and in first position hoist when handling a light hookload.

A still better understanding of the novel features of our invention canbe had from studies of typical operations for both hoisting andloweringl If the attendant wishes to start the motor M, and assumingbuses 0 and 1 are energized, he actuates switch I, whereupon a number ofenergized circuits are established; I

One circuit may be traced from 0 through switch I, conductor 2, fieldcontrol adjustable resistor 3, a fixed resistor 4, rheostat 5, shuntfield winding SH, and conductor 6 to the bus I. The shunt field windingSH, for well known reasons is connected in parallel to a high resistancedischarge resistor 8. The field SH is thus fully en-,

ergized before the motor M is called upon to operate a load.

A circuit is also established from switch I through-conductors 9 and Ileading to the contact finger l l,'which, as shown, remains energizedfor all positions of the controller C. Since all the controllersegments, but one, are interconnected as shown, the segments mayhereinafter be considered positively energized. 0 Other circuits notshown, as for overload and low voltage protection, and some shown arealso established by the operation of switch I, but, except for onecircuit, so far as our invention is con-- cerned have no significance.

,First point hoisting Assuming the attendant wishes to hoist a load 30Another circuit may be traced from the contact finger ll throughconductor l8, coil I9 of line contactor 2|] which thus closes itscontact members 2|. From an inspection of the controller segments it isclear that contactor is in op- 35 erated position for all positions ofthe controller C except for the oii position.

A further circuit is established from contact finger 22, conductor 23,coil 24 of the dynamic braking-control contactor 25 for the control of40 the dynamic braking resistor 26 to the bus I.

The dynamic braking control contactor 25 remains energized for allpositions of the controller except the off position. Since the controlcontactor 25 opens'its contact members 21 the dy- 45 namic brakingresistor 26 is only used at the same timebrake B is, used, namely, foran ofi position of the controller but not always when brake B is used.Final stopping when the controller is moved-from any operating positionto 0 the off position' is thus very rapidly effected. Still othercircuits are established on the first "hoist position. One circuit, onlyeffective for the first hoisting position, may be traced from contactfinger 28 through coil 29 of control con- 55 tactor 30 which closes itscontact members 3| to connect the resistor 32 in parallel with the motorarmature, as will presently appear.

' A circuit is also established from contact finger 33 through coil 34of series circuit control con- 60 tactor 35 and contact members 36 ofanother control contactor 31 to the bus 1. Operation of controlcontactor 35 causes the closing of contact members 38 and 39 and theopening of contact members 40.

65 Another circuit may be traced from contact finger 4|, energized forall controller positions except thelast point lowering, through coil 42of speed control relay 43 to the bus 1. control relay 43 closes itscontact members 44 7 and 45 to thus shunt adjustable resistors 3 and 46.As long'as the load current of the motor M remains fixed below a givenvalue, the shunt field SH will have an adjusted relatively high valuefor all hoisting and lowering controller 7 positions except the lastpoint lowering.

Speed Contact finger 41 remains energized and a circuit may be tracedfrom this contact finger through coil 48 of adjustable inductive timerelay 4,9 to bus 1. This inductive time relay instantaneously opens itscontact members 50 and 5|.

From the operations effected by the circuits traced the motor circuitmay be traced from conductor 2 through the parallel circuit comprisingthe contact members 3|, resistor 32 in parallel with. contact members38, load relay coil 52 and armature A in series with the series field S,resistor sections 53, 54 and 55 and contact members 45 and 2| to the bus1.

At the instant the controller is fully in the first hoist position, themotor speed is zero and since the armature resistance is small most ofthe line current for a loaded motor will pass through the armature. Themotor is, of course, loaded when the slack in the chains and the backlash in the gearing has been taken up and the motor actually begins tolift the load. Before the beginning of such actual lifting of the loadand the motor runs substantially without load because only the chains,the hook, etc., are being operated with the result that substantiallyall the line current, passes through resistor 32. The motor armature isthus subjected only to the voltage across re-.

sister 32. Resistor 32 being of a relatively low resistance and theresistance being also selected in propcr,relation to the resistancecharacteristics of both the armatureA and the series field S, thevoltage on the motor is relatively low with the result that the motorslowly takes up all back lash in the gearing, picks up the chains, thehook, etc., and slowly and gradually without the slightestq'olt picks upthe load on the hook.

Since all the starting current passes through the series field theexcitation will be heavy.

Further, at starting of the load the coil 52 is heavily energized and inconsequence contact members 56 are closed to thus shunt all theresistors,- as 3, 4 and 5, out of the shunt field circuit with theresult that a much heavier field ex' citation is obtained.

Second point hoist Third point hoist On this controller position contactfinger 63 is energized and a circuit is established from this contactfinger through contact members 39, coil 65 of accelerating contactor 66to line 1. Op-

eration of contactor 66 closes contact members 61 to thus shunt resistor55.

Fourth position hoist On this controller position contact finger 68 isenergized and contact finger 51 is deenergized. A circuit is establishedfrom the contact finger 68 through contact members 5|, coil 69 ofaccelerating contactor I0.

It should be noted that contac members 50 and 5| only close after adefinite time interval, since in moving from the second controllerposition to the third contact finger 41 and thus coil 48 of the timelimit relay 49 are deenergized. The attendant thus cannot damage theequipment by a too rapid movement of the controller. Operation ofcontactor I8 closesthe contact members H to thus shunt resistor 58.

Deenergization of contact finger 51 deenergizes time limit relay 58.This relay thus closes its contact members 68, only after a definitetime has elapsed.

Fifthpoint hoist On this controller position contact finger l2isenergized and a circuit is established from this contact fingerthrough contact members, 80 and 50 (if closed by this time), coil I3 ofacceleratin contactor 14 to lines 1. Operation of contactor 15 I4 closescontact members 18 to shunt resistor 53. The controller may, forstopping, be moved slowly successively through all the positions inreverse order in which case all the circuits hereinbefore traced areestablished in reverse order,

but the operation usually desired requires that the controller be thrownto the ofi position.

- The dynamic braking resistance when in the oil position includes theresistor 26 and since the brake also sets in a very short time thestopping is effected rapidly. It should be noted that by our novelarrangement the dynamic braking resistor, contrary to priorart-practice, is only in the motor circuit when needed. This aloneincreases the eificiency' our system over systems of the prior art. Ifthe hook load is light the motor speed is high on the first point hoist,as the controller is moved from any other hoist position to the oil?position,.contact members 8| are closed and a, low

resistance dynamic braking circuit is established through resistor 32 tothus rapidly stop theihook. Another-important feature, not provided byany prior art hoist control. devices, is the provision of a shunt fieldand a special light hook speed adjusting resistor 8. By adjusting theconductor 82, the lighthook speed may be altered at v Lowering Beforeconsidering the detailed operations of our control for load lowering,attention is called to the load relay 83. Since the series field S,v

for lowering, operates as a shunt field, contact members 3i are, as aninspection of controller C shows, closed for all lowering positions andcontact members 38 are open for all lowering,

positions. This means that both coils 82 and 80 are in the armaturecircuit. This means that the load relay is automatically changed in'itscalibration for lowering. For lowering, the armature current is normallylower than for hoisting and both coils are thus used. a

The other circuits that are the same for all lowering positions as forall hoisting pos tions ,need not beagain discussed;

First point 1 In the first lowering position contact fingers 88, 85, 86and" are energized and contact finger l'l'isdeenergized. A circuit isestablished from contact finger 84 to energize the coil 18 of controlcontactor 81 which closes its contact members 18 to establish thearmature cire. cult.

Deenergization of contact'fingerfl effects the deenergization of timelimit relay '88 which clos-v es its contact members 88 and 8| after adefinite time interval. I a

As shown, contact fingers '88 and 88 are shunted by a controller segmentwhereupon a circuit is established immediately from contact v desirablestarting lowering acceleration.-

; ergization of this contact finger for hoisting, coil finger I! throughcontact members 88, contact fingers and 86 to coil 18. Contact members18 are thus'immediately closed. Energization of contact finger\8lestablishes a circuit from this contact finger hrough contact members 8|to 8' coil 68. Contact members it arethus immediately closed. The seriesfield S thus has no resistor sections of the,resistor R in its circuit.The motor is thus heavily excited and if the load is an overhaulingload, that is a heavy load, the operation is to slowly lower the load.Withthe devices of the prior art, on the.first point lowering,-the motorarmature is connected in series with'a dynamic braking resistor directlyacross the series field. The motor, therefore, upon beingsubjected toregenerative braking, is called upon to generate a considerable voltagebefore the necessary braking torque is produced. With our system ofcontrol we do not connect the dynamic. braking resistor in the armaturecircuit, but connect the armature directly across the series field.The'motor, therefore, when subjected to regenerative action, is requiredto generate only a comparatively low voltage to provide the necessarybraking torque. Further, the load relay 83 insures maximum excitationfor the extra field SH. The load may thus be lowered gradually andalmost as slowly as desired without the necessity of any inchingoperation. In short the load can be gently laid down and is not loweredby intermittent short almost free dropping operations as the prior artinching operation provides. This first point lowerin if the load has tobe moved down some considerable distance, thus also provides for a veryslow Second point lowering On the second controller position contactfinger 63 is energized and, as explained for the en- 85 is energized andcontact members 81 are closed. If the load being lowered is heavy or ifthe lowering speed is not to rise or ifboth these conditions obtain theattendant will remain on the first position or at least not move beyondposition two.

In the second lowering controller position'the contact fingers 12, 85,and 88 are deenergized and accelerating contactor I4 is deenergized'andcontact members 15 are opened.

Third point lowering In the third position contact fingers 88 and 81aredeenerg ized and contact finger 51 is energized. Deenergization' ofcontact fingers 88 and 81 opens the circuit for coil 88 of theaccelerating contactor 18. Contact ,members- II open to insert anadditional resistor, namely re-' sister 58 in the field circuit. I

Energization of contact finger 81 energizes coil 88 of time limit-relay'58. This relay thus is-set for future time limit operation.

It should be noted that movement of the controllerto the third positiondeenergized coil 88-. and that in the second position coil. 88 wasenergized. "This means that contact members 8| and 82 are closed. Coil58 is thus energized independent of any segments on the controller.

Fourth point lowering hereinbefore traced. Energization of contact 88finger 88 establishes a circuit from thiscontact finger .through coil 89of the-time limit relay 90 to the bus "I. This time limit relay closesits contact members 93, establishing an additional circuit for coil 58,and opens its contact members 8| and 64. .In this position the circuitfor coil 65 is opened at contact fingers 63 and also at contact members64.

Fifth point lowering In the fifth lowering position the only additionalthing that takes place is the deenergization of contact finger 4i. Coil42 is thus deenergized and in consequence contact members 44 and 45 areopened. The adjustable resistor 46 is insertedin' the field S and theadjustable resistor 3 is inserted in shunt field SH. By the properadjustment of either conductor 82 or 94,

or both, the torque may be adjusted as desired. For an empty hook or forsluggish equipment, or both, or for a loaded hook and sluggish equipmentwe obtain a good starting torque and higlr lowering speed with oursystem.

. speed. Furthermore, the load relay 83 nevertheheavy load, when the lowexcitation is not needlessprotects the motor against excessive speeds inthe event that the overhauling load is above a given value. moves thecontroller to the fifth position for a ed, the load relay preventsexcessive speeds by keeping the excitation of field SH high.

The particular lowering hook speed desired can be obtained by adjustmentof conductor 82 or 94, or both.

Stopping a lowering load The controller may, of course, be slowly movedfrom the fifth or any other position to the 05 position but the usualhabit is to throw the controller to the Ofi position. First fieldcontrol contactor 43 shunts resistors 3 and 46. Accelerating contactor66 will then be energized shunting resistor 55. Thereafter contactor l0shunts resistor 54, and then still later accelerating conselectivedeceleration.

The series field of the motor, during hoisting and when stopping isto'be effected, tends todecrease its excitation and no serious sparkingre-'- tactor I4 shunts resistor 53. We thus provide sults when thedynamic braking resistor is. connected 'across .the armature and seriesfield. However, for lowering and when stopping is to be effected theshiint field. as well as the series field tend to strengthen theexcitation and, in the absence of our special control, serioussparkingwould result, particularly if stopping is to f sparking is prevented bythe provision of our timbe effected from a high operating speed. Thising control 'just discussed.

Still another'important feature thatfurther mcreases the overallefiiciency of motor and control is the; use of the shunt field in thistyp 01' control and the special control for the shun field.

" The low efliciency or prior art series motor con- In other words, itthe attendant trol becomes apparent when the type of control heretoforeused for a series motoris considered.

In order to control a series motor resort must be large current isrequired which causes a considerable 'loss of energy in the seriesresistors.

With the use of a. shunt field as an additional control means and whichfield has many turns the excitation can be very effectively controlledand the energy dissipated in the motor circuit during loweringbecomesvery much less. The efiiciency of our system of control,particularly for the lowering operation is thus considerably higher, butthe overall efiiciency is also much higher, since for every loweringoperation, a corresponding hoistingoperation has to be performed. q

The curves f, g, h, i, j, k, l, m, n and 0 of Fig.

2 show that there is good distribution of the curves, for the varioushoisting and lowering positions thus eliminating jerking of the 'load inthe process of speeding up or slowing down. I As hereinbefore stated thedynamic braking reslstor is. only in the circuit when needed. This givesa more even speed as can be seen from the flatness of the curves (seeFig. 2), particularly 'for the lowering direction. point of thecontroller, the speed will vary less with variations in load than forprior art control schemes due to, theabsence of the braking resistorfrom the armature circuit.

we are, of course,- aware that others skilled in the art andparticularly after having had the benefit of our teachings can deviseslmilar syse' tems of control for accomplishing the novel results hereinpointed out. wish to be limited to the particular circuits shown anddescribed but wish to be limited only by the pertinent prior art and theclaims hereto appended. r

We claim as our invention:

1. In a system otcontrol for a direct current motor for operating ahoist as a cargo winch, in combination, a motor, an armature winding anda field winding therefor, means for selectively On any lowering We,therefore, do not 7 connecting the field winding'either in'series withthe armature winding or in parallel with the armature winding, a secondfield winding for the altering the resistance value of the resistance tothus automatically control the current .value in the second fieldwinding.

2.:In a system 01' control for a direct current motor for operatingahoist, as a cargo winch, in combination, a motorg'an armature windingand a field winding therefor, means for selectively interconnecting thearmature winding and field winding so that said windings are for oneselected interconnection in parallel and for another selectedinterconnection in series, a load current responsive device operable atone current value for one selected interconnection and operable atanother-current value for anothen selected interconnection, and .asecond field winding for the motor disposed'to be controlled by saidload current responsive device.

3. In a system of control for a direct current controller adapted forone set of operating conditions to connect the field winding in serieswith the armature winding and for another operating condition inparallel circuit with the armature winding, an armature currentresponsive device controlled by said master controller to be responsiveto operate at difierent current values depending upon which set ofoperating conditions of the master controller is selected, a secondfield winding for the motor resistance means, the resistance value ofwhich may be altered, interconnected with said second field winding, andmeans responsive to the operation of the armature current responsivedevice for altering the resistance value of said resistance means thusto control the excitation of the second field winding.

4. In a system of control for a direct current motor for operating ahoist, as a cargo winch,

in combination, a direct current motor, an armature winding'and afieldwinding therefor, a source of direct current energy, selectiveswitching means for connecting said field winding either in series withthe, armature winding or in shunt relation to the armature, means forconnecting the armature winding and field winding to the source ofenergy, an armature shunt circuit,and means for connecting the armatureshunt circuit across the armature when the selective switching means isin such position as to connect the field winding and armature winding inseries whereby a low voltage is impressed onthe armature winding whenthe source of energy is connected to the armature winding'andfieldwinding. 5. In asystem of control fora direct curren motor foroperating a hoist, as a-car'go winch, in combination, a direct currentmotor having an armature and a field winding, a source of direct currentenergy, selective switchingmeans for connecting said field .winding'either in series with the armature winding or in shunt relation to thearmature, means for connecting the armature winding and field winding tothe source of energy, an armature shuntcircuit, means for connecting thearmature shunt circuit across the armature winding when the selectiveswitching means is insuch position as to connect the field winding andarmature winding in series whereby a low voltage is impressed on thearmature winding when the. source of en-.

ergy is connected to the armature winding and field winding, and meansfor exciting the field winding a maximum at the time the armature issubjected to a low voltage.

6. In a system of control for a direct current motor for-operating ahoist, as a cargo winch, in

combination, a direct current motor having an armature winding andafield winding, a source of direct current energy. elective switchingmeans for connecting said 'field winding either in series with thearmature winding or in shunt relation-to the armature means forconnecting the armature winding and field winding to thesouroe ofenergy, an armature shunt circuit, means for connecting the armatureshunt I circuit across the. when the selective switching means is insuch position as to connect the field and armaturewindinseries'wherebya. low voltage is impressed -on the armature winding when the source ofen- '7. In a system of control for a direct current motor for operatinga hoist, as a cargo winch, in

combination, a direct current motor having an armature winding and afield winding, a source of direct current "energy, .selective switchingmeans for connecting said field winding either in series with thearmature winding'or in shunt relation to the armature, means forconnecting the armature winding and field winding to the source ofenergy, an armature shunt circuit, means for connecting the armatureshunt circuit across the armature Winding when the selective switchingmeans is in such position as to connect the field winding and armaturewinding in series whereby a low voltage is impressed on the armaturewinding when the source of energy is connected to the armature windingand field winding, means for exciting the field winding a maximum at thetime the armature is subjected to a low voltage, means independent ofsaid field winding for also providing a component of magneto-motiveforce I in the motor at a given current in the armature winding, andmeans for altering the operating characteristic of the last named meansas a function of the operation of said selective switching means.- I

8. In a system of control for a motor operating a hoist, in combination,a direct current motor, said motor having a field winding and anarmature winding, a circuit controller adapted for one set of operatingconditions to connect said fieldv winding and armature winding inseries, and adapted for another set of operating conditions to connectsaid field winding and armature winding in parallel, a separate fieldwinding for also exciting the motor, and a. load current responsiverelay for controlling the excitation of the separate field winding.

9.In a system of control for a motor operating a hoist, in combination,a direct current motor, said motor having alfield winding and anarmature winding, a manually operable circuit.

ing' a hoist, in combination, a direct current motor, said motor havinga field winding and an armature winding, a manually operable circuitcontroller having a plurality of" operating posi-' tions for efiectingoperation of the motor in one direction,'saidxcircuit controllerconnecting the motor armature winding and field winding in series andfor successive operating positions successively decreasing theresistance inthe arma- .ture and series connected fleldwinding circuit,

a separate fieldior also exciting the motor, and means responsive to.the armature current of the motor for controlling the excitation effectof the separate field winding. F

11; In a system of control for a motor operating a hoist, incombination, a direct current motor, said motor having a field windingand an armature winding, a manually operable circuit controller having aplurality of operating positions, said circuit controller connecting themotor armature winding and field winding in series and for successiveoperating positions successivel'y decreasing the resistance in thearmature and series connected field winding circuit, a resistorconnected in shunt relation to the armature winding on the firstoperating position of the controller, a separate field for also excitingthe motor, and means responsive to the armature current of the motor forcontrolling the excitation effect of the separate field winding.

12. In a system of control for a motor operating a hoist, incombination, a direct current motor, said motor having a field windingand an armature winding, a manually operable controller having one groupof operating positions for connecting the armature winding and fieldwinding in parallel and having another group of operating positions forconnecting the armature winding and thefield winding in series, aseparate field winding connected in shunt relation to the armaturewinding and first named field winding, and

- means, operable upon actuation of the controller field windingconnected in shunt relation to the armature winding and first namedfield winding, a load,,or armature, current responsive relay forcontrolling the excitation effect of the separate field winding, andmeans, operable upon actuation of the controller to its last position ofthe first group of operating positions, for effecting a decrease of theexcitation of both fields windings to a minimum.

14. In a system of control for a motor operat, ing a hoist, incombination, a direct current motor, said motor having a field windingand an armature winding, a manually operable controller having one groupof operating positions for connecting the armature winding and fieldwinding in parallel and having another group of operating positions forconnecting the armature winding and the field winding in series, aseparate field winding connected in shunt relation to the armaturewinding and first named field winding, means, operable upon actuation ofthe controller to its last position of the first group of operatingpositions, for decreasing the excitation of both field windings to aminimum, and means for adjusting such minimum excitation to which theexcitation of both field windings is decreased.

15. In a system of control for a motor operating a hoist, incombination, a direct current motor, said motor having a field windingand an armature winding, a manually operable controller having one groupof operating positions for connecting the armaturewinding and fieldwinding in parallel and having another group of operating positions forconnecting the armature winding and the field winding in series, aseparate field winding connected in' shunt relation to the armaturewinding and first named field winding, a load, or armature, currentresponsive relay for controlling the excitation efiect of the separatefield winding, means, operable upon actuation of the controller to itslast position of the first group of operating positions, for effecting adecrease of the excitation of both field windings to a minimum, andmeans for adjusting such minimum excitation to which the excitation ofboth field windings is decreased.

16. In a system of control for a motor operating a hoist, incombination, a direct current motor, said motor having a field winding ad an armature winding, a circuit controller for conmeeting said fieldwinding and armature winding in series for one set of operatingconditions, and

motor armature to provide a low voltage on the armature and means forsimultaneously heavily exciting both field windings to thus provide aslow motor speed and heavy starting torque.

17. In a. system of control for a motor operating a hoist, incombination, a direct current motor, said motor having a field windingand an armature winding, means for connecting the armature windingdirectly in parallel to the field winding whereby a large braking torqueis de-' veloped by the motor when subject to regenerative action, adynamic braking resistor connected to the armature winding only when thearmature winding is not energized from a source of energy. WALTERSCHAELCHLIN.

- KURT MAHNKE.

